Why not dilute radioactive waste? Radioactive wastes are dangerous because unstable elements are too concentrated. Originally radioactive elements come from nature where they were very diluted and that's why they were secure. So why dilution of radioactive waste back in to the nature is never mentioned as waste treatment ?
 A: Probably too expensive and disruptive to try to deal with nuclear waste that way. You're talking about processing through an enormous amount of earth and/or seawater. Note that nuclear waste includes not just material that was initially radioactive when it came out of the ground (e.g., uranium ore), but a lot more material as well. If a nuclear plant worker touches some rods filled with uranium, for example, those gloves also become (low level) nuclear waste since they can't very well throw such gloves into a regular trash can, can they? Also with the advent of breeder reactors it's been possible to make more highly radioactive material than was initially mined out of the Earth, so you're talking about not only putting back a lot of radioactive material that we initially got from the Earth but a tremendous amount more besides. Bottom line is that trying to grind or pulverize all that material and then apply massive dilution with lots or soil and/or seawater is probably not a very convenient or cost-effective option.
A: One on the problem is re-concentration, by the help of water circulation in the soil (possibly up to water sources) or by the help of small animals (then to food chain up to us). 
The stability of geological layers is not so easy to predict.
Beside, the radio-activity of wastes can be a lot higher, and spreaded through a huge variety of chimical species, with each further change might react differently to environmental conditions.
Also there is a responsability issue: it's more easy to ensure quarantaine in a limited volume. Nobody would like to hold the garantee that spreading diluted waste everywhere will have absolutely no consequence. Populations accept artifical risks a lot less than natural risks !
A: 
Originally radioactive elements come from nature where they were very diluted and that's why they were secure. 

When these naturally radioactive materials like Uranium are used in processes like civilian nuclear energy production the resulting waste becomes many, many times more radioactive than the raw materials one started off with. Even after the cooling off period the waste contains many dangerously radioactive materials that would be very toxic for human and other life forms, even in very low quantities.

[...] dilution of radioactive waste back in to the nature is never mentioned as waste treatment

... because it would constitute an unprecedented ecological disaster.
